Nutrient-uptake in experimental estuarine ecosystems: scaling and partitioning rates

Studies of nutrient cycling and enrichment in aquatic ecosystems are common ly conducted in enclosed experimental ecosystems. Although there is conside rable information about how the dimensions of natural aquatic ecosystems in fluence nutrient cycling processes, little is known on how nutrient cycling studies might be affected by the physical scales of experimental enclosure s. In the present study, replicate (n = 3) cylindrical containers of 5 dime nsions with 3 volumes (0.1, 1.0, 10 m(3)), 3 depths (0.46, 1.0, 2.15 m), an d 5 diameters (0.35, 0.52, 1.13, 2.44, 3.57 m) were established and subject ed to pulsed additions of dissolved inorganic nutrients (DIN, PO43-, Si) in summer and autumn experiments. Consistent with common experimental protoco ls, walls of these containers were not cleaned of periphytic growth during the 8 wk studies. Nutrient concentrations in experimental ecosystems were l ow prior to nutrient-pulse additions and exhibited exponential depletion fo llowing treatments. Overall, larger containers had lower net uptake rates a nd higher nutrient concentrations than did smaller tanks. Relative contribu tions of planktonic, benthic and wall periphytic communities to total nutri ent uptake varied in relation to dimensions of experimental systems. In gen eral, net uptake rates by planktonic communities were inversely related to water depth, with higher rates associated with increased mean light-energy in shallower systems. Indirect estimates of benthic uptake rates, which wer e relatively low in all but the shallowest systems, tended also to be inver sely related to depth and directly proportional to light levels at the sedi ment surface. In contrast, nutrient uptake by wall communities (per water v olume) was inversely related to the radius of experimental containers. Diff erences in the 2 container dimensions, depth and radius, accounted for more than 90% of the variance in both net nutrient uptake by the whole ecosyste m and the molar ratio of DIN/PO43- concentrations in the water column. Simi larly, differences in net nutrient uptake rates among experimental ecosyste ms of different dimensions could be explained by the relative partitioning of rates among planktonic, periphytic, and benthic habitats. These results demonstrated that the physical dimensions of experimental ecosystems can ha ve profound effects on measured nutrient dynamics. We also suggest that man y of these experimental observations may be relevant also to more general s caling relations for nutrient cycling in natural aquatic ecosystems.